1. Technical Field
The present disclosure relates to a planar microelectromechanical device having a stopper structure for out-of-plane movements.
2. Description of the Related Art
Microelectromechanical devices (of a MEMS type), in particular inertial sensors, accelerometers, gyroscopes, etc., are known which base their operation on the presence of suspended structures of semiconductor material, fixed to a substrate at one or more anchorage points and mobile along one or more axes. In particular, the suspended structures form one or more mobile masses, which undergo displacements with respect to the substrate in presence of external stimuli. In the case of sensing structures (for example, accelerometers), first electrodes are associated with the mobile masses and face second electrodes, which are fixed with respect to the substrate, so as to form sensing capacitors, the capacitance of which varies with the displacement of the mobile mass. A suitable electronic interface is able to detect, on the basis of the variation of said capacitance, the displacement of the mobile mass, and thus to go back to a quantity to be determined (for example, an acceleration).
FIG. 1 is a schematic illustration of an embodiment of a planar microelectromechanical device, designated by 1 (in particular, by the term “planar” is meant herein a structure that extends principally in a plane xy and is obtained with a substantially planar manufacturing process). The microelectromechanical device 1 comprises a mobile mass 2, of an inertial type, suspended over a substrate 3, for example, of semiconductor material, and connected to a fixed element 4 via elastic elements 5. The fixed element 4 is fixedly anchored to the substrate 3 via an anchorage region 6, arranged between the substrate and the fixed element. The elastic elements 5 are configured so as to enable the mobile mass 2 to perform an operative movement in one or more directions, for example, along a first axis x and a second axis y orthogonal to one another and defining the xy plane, for example as a function of an acceleration acting on the structure.
In a known way, in use, on account of an external event, for example a shock, the mobile mass 2 can moreover undergo a spurious displacement (i.e., one not linked to operation of the structure) of a considerable amount outside the plane xy along a third axis z, orthogonal to the first and second axes x, y and forming therewith a set of three Cartesian axes. This displacement can, in the worst case, even cause failure of the elastic elements 5, and/or damage to other elements of the microelectromechanical device 1 (for example, the mobile mass 2).
For this reason, a stopper structure 7 is generally provided to limit the movements of the mobile mass 2 along the third axis z, in particular the spurious movements due to external events, and thus prevent any undesirable failure. In detail, the microelectromechanical device is enclosed in a package 8, and has at least one internal face 8a facing a top face 2a of the mobile mass 2. The stopper structure 7 comprises a depression of the package 8, which is located on the internal face 8a and extends towards the top face 2a of the mobile mass 2. The stopper structure 7 reduces the range of the movement along the third axis z of the mobile mass 2, which comes to a stop bearing upon the depression of the package, thus reducing any risk of damage.
However, the depression, which is located close to the mobile mass 2 (in order to limit the range of its movement along the third axis z), gives rise to an undesirable electrostatic interaction with the mobile mass, which can cause undesired displacements thereof and in general malfunctioning of the microelectromechanical device 1. In fact, the package (and consequently the depression coupled thereto) are generally maintained either at a floating potential, and hence not at the same potential as the mobile mass, or at a fixed potential, while the mobile mass is supplied with different potentials according to the operating condition. In any case, the undesired displacements can alter operative displacements of the mobile mass 2 that occur in response to external forces (for example, an acceleration that is to be determined). On the other hand, since the manufacturing process of the microelectromechanical device is a basically planar process, where it is simple to create structures in the xy plane, is not equally simple to obtain them along the third axis z, so that up to now there do not exist valid alternatives to the use of depressions in the package.